Further, cast iroii weighing 450 Ibs. per cub. ft.,
wt. of fly-wheel . . . w — ^D2 x — x 450
3 = DS
and R« -
and wR2 = I
» = 87514
A D5 « 87514x8 ^ 20000 nearly
And D = V 20,000 = 7^ feet.
Pp. 4?8and68o. Moment of Inertia by Experiment.
—The method to be here described is especially suitable for
cylindrical objects such as fly-wheels, pullies, lines of shafting,
and the like; though also usable in the case of other objects if
placed within a ring for the purposes of the experiment. Referring
to Fig. 972, a strap or cord passes round the rim of the object
and supports two experimental weights Wl and W2, of which Wl is
always heavier. Imagine the mass M of the body to be concen-
trated at the rim, and the frictional force F to be similarly treated.
Also R0 = radius at rim, and R the radius of gyration. Of course
Wx begins to descend with uniform acceleration f> the force being